MODERN HEAT TREATMENT OF FERROUS ALLOYS WITH AN ISOTHERMAL STOP Kamil Wasiluk, Emilia Skołek, Karolina Szwejkowska, Wiesław Świątnicki High mechanical properties of steel are most often obtained by heat treatment, specifically hardening and tempering. The martensitic hardening process dramatically increases the hardness and strength of steel, but at the same time it reduces plasticity and brittleness. In addition, it produces high stress in the material and, consequently, changes the dimensions of the hardened element. In order to improve the plasticity, the tempering process is applied after hardening, although increasing the tempering temperature increases the plasticity but decreases the strength. Unfortunately, the tempering process does not reduce hardening deformations. A good combination of high hardness and mechanical properties as well as impact strength and plasticity can be achieved through the use of the thermal treatment, isothermal quenching. This treatment, unlike continuous hardening with martensitic transformation, most often uses bainitic transformation. The isothermal quenching is based on austenitization followed by rapid cooling to bainitic transformation temperature and remaining at this temperature for the time needed to complete the bainite conversion and final cooling to ambient temperature (Figure 1). It allows the formation of a lower bainite microstructure, comprising of carbon oversaturated bainite ferrite slabs with eductions of cemantyte. In ADI (Austempered Ductile Iron) cast iron, this treatment is used to INTRODUCTION